Polyolefin compositions with improved impact strength

ABSTRACT

The addition of a polyolefin-acrylic graft copolymer to blends of polyolefin and core-shell polymers improves compatibility and allows core-shell polymers to be use as processing and performance modifiers polyolefins. Generally the compatibilizing additive is a graft copolymer of a polyolefin and a methacrylate. More specifically, the graft copolymer is derived from at least about 80% of a monomer of a methacrylic ester of the formula CH 2  ═C(CH 3 )COOR, where R may be alkyl, aryl, or aralkyl, substituted or unsubstituted, and less than 20%, based on a total monomer weight, of an acrylic or styrenic monomer copolymerizable with the methacrylic ester grafted on to a non-polar polyolefin trunk, so that at least one chain is a polymer with a weight average molecular weight greater than about 20,000 and is present in a weight ratio with the polyolefin trunk of from about 1:9 to about 4:1. 
     A typical composition may contain about 20% core-shell modifier, about 10% compatibilizer, the balance being polyolefin.

This is a divisional of application Ser. No. 413,824, filed Sep. 28,1989 now U.S. Pat. No. 4,997,884 which is a Continuation-In-Part of U.S.Ser. No. 07/315,501, filed Mar. 1, 1989, the disclosure of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to polyolefins with improved impact strength,obtained by the addition of core-shell polymers compatibilized to thepolyolefin matrix via the addition of a polyolefin-acrylic graftcopolymer.

2. Description of Related Art

Polyolefins are widely used as general purpose resins, however, from theperspective of practical physical properties, suffer from having lowimpact strength.

Core-shell polymers are known as useful additives for certain plastics,providing both processing and end use benefits. As processing additivesthe core-shell polymers allow the melt to be processed more quickly andwith better consistency by improving the melting and increasing the meltstrength. As performance additives the core-shell polymers increase theimpact strength of plastics.

However, it has not been possible to use core-shell polymers inpolyolefins since the polyolefins are non-polar and are, therefore,non-compatible with the polar core-shell polymers.

U.S. patent application Ser. No. 315,501, of common ownership herewith.,describes the preparation of novel grafted copolymers capable ofimparting to polyolefins when blended therewith high tensile modulus andhigh sag resistance. The graft copolymers of this disclosure arepolyolefins having a relatively high weight average molecular weightmethacrylate polymer grafted thereto.

Fayt and Teyssie in Polymer Engineering and Science, Apr. 1989, Volume29, Number 8, pages 538 to 542, disclosed the emulsifying activity of acopolymer of . poly (hydrogenated butadiene-β-methyl methacrylate) as anemulsifier for blends of low density polyethylene and polystyrenecoacrylonitrile resins.

U.S. Pat. No. 4,430,477 discloses high impact polypropylene compositionscomprising polypropylene and a chlorinated polyolefin having styrenegraft polymerized thereon.

U.S. Pat. No. 4,156,703 discloses the blending of a polyolefin, namelypolyethylene with an acrylic polymer to form a heterogeneous mixture,which has improved processing characteristics.

U.S. Pat. No. 4,180,494 discloses a core-shell polymer comprising about25 to 95% by weight of an aromatic polyester, about 1 to 8% by weight ofan aromatic polycarbonate, and a balance to make 100% of the core-shellpolymer having a butadiene based core.

U.S. Pat. No. 4,443,585 discloses polymer impact modifiers usedespecially for PVC. The impact modifiers of this disclosure arecomprised of an olefin and an acrylic copolymer.

U.S. Pat. No. 3,808,180 discloses modified rigid acrylic thermoplasticresin comprising at least 50 weight percent alkyl or aralkyl acrylate,crosslinking and hydrophilic monomers. These materials may becharacterized as soft-core hard-shell polymers.

U.S. Pat. No. 3,793,402 discloses an impact resistant thermoformablecomposition comprising a blend of a thermoplastic polymer and ahard-soft-hard core additive.

U.S. Pat. No. 4,617,329 discloses a core-shell polymer wherein the coreis polymerized from a styrenic monomer. The second stage comprises apolymeric soft stage with a rigid thermal plastic outer stage.

The disclosures of the foregoing patents and publications areincorporated by reference herein. It is the object of this invention toincorporate the core-shell polymers as described above into polyolefins.

SUMMARY OF THE INVENTION

The addition of a polyolefin-acrylic graft copolymer to blends ofpolyolefin and core-shell polymers improves compatibility and allowscore-shell polymers to be used as processing and performance modifiersin polyolefins. Generally the compatibilizing additive is a graftcopolymer of a polyolefin and a methacrylate. More specifically, thegraft copolymer is derived from at least about 80% of a monomer of amethacrylic ester of the formula CH₂ ═C(CH₃)COOR, where R may be alkyl,aryl, or aralkyl, substituted or unsubstituted, and less than 20%, basedon a total monomer weight, of an acrylic or styrenic monomercopolymerizable with the methacrylic ester grafted on to a non-polarpolyolefin trunk, so that at least one chain is a polymer with a weightaverage molecular weight greater than about 20,000 and is present in aweight ratio with the polyolefin trunk of from about 1:9 to about 4:1.

A typical composition may contain about 20% core-shell modifier, about10% compatibilizer, the balance being polyolefin.

DETAILED DESCRIPTION OF THE INVENTION

The preparation of the compatiblizing additives useful in this inventionis described in detail in U.S. Ser. No. 315,501, the disclosure of whichis incorporated by reference herein.

Briefly, the additive useful in this invention is prepared by graftingonto a non-polar polyolefin trunk in solution, at least one chain whichis of a polymer having a weight average molecular weight (Mw) of fromabout 20,000 to about 200,000 and present in a weight ration with thepolyolefin of from about 1:9 to about 4:1.

The molecular weight of the polyolefin polymer which forms the trunk ofthe graft copolymer should be high enough to give a large amount ofnon-polar polymer when grafted, but low enough so that most of the graftcopolymer has one acrylic polymer chain grafted to each polyolefin trunkchain. The trunk may have a molecular weight between about 50,000 and1,000,000. The trunk may also have a molecular weight of about 100,000to 400,000. A polyolefin trunk having a molecular weight of about200,000-800,000 Mw is especially preferred, but polyolefins having amolecular weight of about 50,000-200,000 can be used with somebeneficial effect.

In using the compatibilizing additives of this invention it is possibleto melt-mix all three components, i.e. the polyolefin, the core-shellmodifier and the compatibilizer simultaneously in an extruder.Alternatively the compatibilizer and the modifier may be melt-mixed andthen added to the polypropylene or other polyolefin. Or a blend of themodifier and the compatibilizer may be added to the moltenpolypropylene.

In the practice of this invention compatibility is used as a criterionfor determining if an acceptable mixture has been produced. Compatiblepolymers tend to form blends that contain small domains of theindividual polymers; in the case of miscible polymers these occur at themolecular scale resulting in properties usually consideredcharacteristic of a single polymer. These may include occurrence of asingle glass transition temperature and optical clarity. Such blends arefrequently termed alloys.

Compatible polymers that are not strictly miscible, as described above,nevertheless tend to form blends with properties that approach those ofthe miscible blends. Such properties as tensile and impact strength,which rely upon adhesion of the domains to one another, tend not to bedegraded when compatible polymers are blended.

Poor compatiblity cannot necessarily be predicted accurately for a givenpolymer combination, but in general it may be expected when non-polarpolymers are blended with more polar polymers. Poor compatibility in ablend is apparent to those skilled in the art, and often evidencesitself in poor tensile strength or other physical properties, especiallywhen compared to the component polymers of the blend. Microscopicevidence of poor compatibility may also be present, in the form oflarge, poorly-adhered domains of one or more polymer components in amatrix of another polymer component of the blend. More than one glasstransistion temperature may be observed, and a blend of otherwisetransparent polymers may be opaque because the domain sizes are largeenough to scatter visible light. As a rule of thumb, 5 micron particlesare an indication of incompatibility. The incompatibility will often bemanifested by delamination of the materials in preparation. When thedomain sizes are on the order of 0.5 micron or less, compatibility hasbeen achieved.

Among the olefins which are expected to benefit from application of thiscompatiblizing technology are polyethylene, polypropylene,poly-1-butene, polymethylpentene, EPDM terpolymers and copolymers of theabove. It is also expected that mixtures of these polymers in normalratios would be suitable in the practice of this invention.

Among the core-shell modifiers which are expected to find use in thisinvention are MBS and acrylic modifiers, such as butadiene/methylmethacrylate and butyl acrylate/methyl methacrylate materials. Typicallythe composition may contain anywhere from 0.1 to 40 parts of core-shellmodifier. The ratio of graft copolymer compatibilizer to modifier willgenerally be in the range of about 1:10 to about 1:1.

The products may be isolated by stranding, cooling, chopping, drying andbagging or other known collection techniques. Optionally the blend ofpolyolefin, impact modifier, and compatiblizer may be further modifiedby the introduction of fillers, both organic and inorganic, fibers,other impact modifiers, colorants, stabilizers, flame-retardants, and orblowing agents.

The blend of polyolefin, impact modifier and compatibilizing additive isuseful in thermo-forming, film-making (especially blowing andextruding), blow-molding, fiber-spinning, foaming, extrusion (sheet,pipe, and profile), co-extrusion (multi-layer film, sheet, preforms, andparisons, with or without the use of tie-layers), hot-melt adhesives,calendering, and extrusion coating (for the preparation ofpolymer/fabric, carpet, foil, and other multi-layer constructions).

When polypropylene is modified with the core-shell modifier andcompatibilizing additives of this invention, the resultant mixture maybe employed in the manufacture of many useful objects, such as extrusionor injection-blown bottles for the packaging of food-stuffs, aqueoussolutions such as intravenous feeds, hot-filled items such as ketchup,or extruded articles in profile forms such as clips, scrapers, windowand door casings and the like. Foamed articles may be used assubstitutes for wood in moldings, for packaging materials, forinsulation or sound-deadening materials, for food containers, and otherrigid article applications. Films may be used in many protective orwrapping applications such as food packaging, blister packaging ofconsumer goods, and the like.

The compositions of this invention are also useful in preparation ofpolyolefin fibers, especially polypropylene fibers. Polypropylene may beformed into fibers by slitting tape from extruded film to form highdenier, coarse fibers; by extruding monofilaments into large denierfibers with a controlled cross-sectional size, or by extrudingmultifilaments through a spinerette to produce bundles of small denierfibers. In all cases, the fibers may be draw-textured.

The following examples are intended to illustrate the present inventionand should not be construed as limiting the invention to less than thatspecified by the claims. All percentages are by weight unless otherwisespecified and all reagents are of commercial quality.

EXAMPLE 1 Preparation of Graft Copolymer of Methyl Methacrylate, EthylAcrylate and EPDM

An EPDM- acrylic graft copolymer was made by polymerizing an 8% ethylacrylate-92% methyl methacrylate monomer mixture in the presence of anequal amount of EPDM (a copolymer of ethylene, propylene, and anon-conjugated diene, such as 1,4 hexadiene). Radicals were generatedfrom di-tertiary-butyl peroxide (DTBPO) at the rate of 0.00017 mols perliter per minute (radical flux). Monomer and initiator were fed over 30minutes. The theoretical (100% conversion) solids at the end of thereaction is 55%.

A 1.7 gallon reactor equipped with a double helical agitator (115 RPM)was charged with 1780 grams of Isopar E (an inert aliphatic hydrocarbonsolvent mixture) and heated to 150° C. Eleven hundred grams of an EPDMpolymer (DuPont Nordel 2722) was fed to the reactor via melt extruderset at 150° C. at a rate of about 10 grams per minute. After holding for45 minutes at 170° C. the addition of monomer and initiator solutionswas begun. Over a 2 minute period two solutions were added. The firstconsisted of 1.9 gram of di-t-butyl peroxide in 20 gram of Isopar E. Thesecond consisted of 0.07 gram of di-t-butyl peroxide in 5.9 gram ofethyl acrylate and 68 grams of methyl methacrylate. Over the next 28minutes, 0.99 grams of di-t-butyl peroxide and 82 grams of ethylacrylate in 946 grams of methyl methacrylate were added.

This feed schedule results in a radical flux of 0.00017 during the feed.After the feed was complete the reaction was held at 150° C. for anadditional 15 minutes. It was then devolatilized by passing through a 30mm Werner-Pfleiderer extruder with vacuum vents at 200°-250° C. Theelemental analysis (carbon content) showed that the composition was 44%acrylate and 56% EPDM.

EXAMPLE 2 Preparation of Graft Copolymer of Polypropylene, MethylMethacrylate, and Ethyl Acrylate

A polypropylene-acrylic graft copolymer was made by polymerizing a 5%ethyl acrylate-95% methyl methacrylate monomer mixture in the presenceof polypropylene (weight ratio of polypropylene to monomer equals 0:67to 1. Radicals were generated from di-t-butyl peroxide at the rate of0.00010 mols per liter per minute (radical flux). Monomer and initiatorwere fed over 60 minutes and the theoretical (100% conversion) solids atthe end of the reaction is 55%.

A 1.7 gallon reactor equiped with a double-helical agitator (115 RPM)was charged with 1780 grams of isopar E and 880 grams of polypropylene(Himont 6523) and heated to 175° C. After two hours the temperature wasdecreased to 155° C. and the batch was stirred for one additional hour.Over a 2-minute period two solutions were added. The first consisted of1.03 grams of di-t-butyl peroxide in 21 grams of Isopar E. The secondconsisted of 0.06 grams of di-t-butyl peroxide in 2.1 gram of ethylacrylate and 42 gram of methyl methacrylate. Over the next 58 minutes,1.87 gram of di-t-butyl peroxide and 62 gram of ethyl acrylate in 1215gram of methyl methacrylate were added. This feed schedule resulted in aradical flux of 0.00010 during the feed time.

After the feed was complete, the reaction was held at 155° C. for anadditional 15 minutes. It was then devolatilized by passing through a 30mm Werner-Pfleiderer extruder with vacuum vents at 200° to 250° C.Elemental analysis showed that the composition was 56% acrylate and 44%polypropylene.

EXAMPLE 3 Preparation of Graft Copolymer of Polypropylene, MethylMethacrylate, and Ethyl Acrylate

A polypropylene-acrylic graft copolymer was made by polymerizing a 5%ethyl acrylate-95% methyl methacrylate monomer mixture in the presenceof polypropylene (weight ratio of polypropylene: monomer=0.67:1).Radicals are generated from di-t-butyl peroxide at the rate of 0.000065mols per liter per minutes (radical flux). Monomer and initiator are fedover 122 minutes and the theoretical 100% conversion solids at the endof the reaction is 47%.

A 100 gallon reactor equiped with a back-slope turbine agitator wascharged with 225 pounds of Isopar E and 80 pounds of polypropylene(Hercules 6523PP) and heated to 150° C. over four hours. Two solutionswere added over a 20-minute period. The first consisted of 82 gram ofdi-t-butyl peroxide in 826 gram of Isopar E. The second consisted of 1pound of ethyl acrylate and 19 pounds of methyl methacrylate. Anadditional 82 grams of di-t-butyl peroxide and 82 grams of Isopar E wereadded over 90 minutes. At the same time, 5 pounds of ethyl acrylate and95 pounds of methyl methacrylate were added over 102 minutes (ending 12minutes after the initiator feed had finished). The reaction was held at150° C. for an additional 15 minutes. Then an additional 50 pounds ofIsopar E were pumped in over 30 minutes. The reaction mixture was thendevolatilized by passing through a 0.8 inch Welding Engineer's twinscrew extruder at 200 RPM and 200° to 250° C. over 14 hours. Elementalanalysis showed this copolymer to be 45% polypropylene and 55% acrylate.

EXAMPLE 4 Blend of Polypropylene, Core-shell Modifier, andCompatibilizer of Example 1

This example shows the effect of blending core-shell modifiers intopolypropylene with compatibilizing additive.

Sixty grams of a copolymer, butadiene/methyl-methacrylate (Rohm and HaasKM-680) and 30 gram of the EPDM-acrylic graft copolymer of Example 1were blended on a 3 by 7 inch electric mill at 190° C. for 3 minutesafter all material had fluxed. After this material had cooled, 30 gramsof it were mixed with 70 grams of polypropylene (Himont 6523) on themill at 190° C. for 3 minutes after flux. The material was then pressedin a 0.125×5×5 inch mold in a Farrel press at 190° C. and 75 tonspressure. The mold was then placed in a cooling press at 75 tons for 2minutes. Notched Izod impact strength of a specimen cut from this sheetwas 0.52 ft-lb/in.

EXAMPLE 5 Blend of Polypropylene, Core-shell Modifier, andCompatibilizer of Example 2

In a similar manner to that of Example 4, the graft copolymer of Example2 was blended with polypropylene. The notched Izod impact strength of aspecimen cut from this sheet was 0.46 ft-lb/in.

EXAMPLE 6 Blend of Polypropylene and Core-shell Modifier

A specimen was prepared with only core-shell polymer in polypropyleneand the appearance of the blend indicated that these materials were notcompatible. Notched Izod impact strength for a specimen cut from thissheet wa only 0.34 ft-lb/in.

EXAMPLE 7 Virgin Polypropylene

A sheet of an unmodified polypropylene control with no additives wasprepared in a similar manner. A specimen of this sheet had a notchedIzod impact strength of 0.41 ft lb/in.

EXAMPLE 8 Blend of Polypropylene, Multi-stage Processing Aid, andCompatibilizer

A blend of 10 grams of K120-ND melt processing aid and 35 grams of thepolypropylene acrylic graft copolymer of Example 3 was prepared in themelt in a Haake Rheocord at 180° C. This mixture was then melt-blendedwith polypropylene, 2.25 grams of the mixture to 42.75 grams ofpolypropylene. A control blend of the K120-ND and polypropylene wassimilarly prepared.

Morphological observation via transmission electron microscopy showedthat the K120-ND blend without the graft copolymer had very largeirregular shaped domains of undispersed acrylic, typical of incompatiblepolymer blends.

The sample containing graft copolymer had far superior dispersion. Theacrylic domains were spherical and varied in size from 0.1 to 0.5microns, indicative of improved compatibility.

While this invention has been described with reference to specificexamples and applications, other modifications and uses for theinvention will be apparent to those skilled in the art without departingfrom the spirit and scope of the invention defined in the appendedclaims.

What is claimed is:
 1. A process for preparing a polymeric blend withimproved impact resistance comprising melting a polyolefin, adding anacrylic core-shell modifier to said polyolefin melt, adding apolyolefin-graft copolymer compatibilizer to said melt, said polyolefinacrylic graft copolymer comprising:(i) a non-polar polyolefin trunkselected from the group consisting of polyethylene, polypropylene,polybutylene, poly(4-methylpentene), copolymers of said olefins witheach other, and one or more copolymers of said olefins with minoramounts of 1-alkenes, vinyl esters, vinyl chloride, (meth)acrylic ester,and (meth)acrylic acid, said trunk having a Mw of between about 50,000and 1,000,000; and (ii) at least one methacrylate chain grafted with acovalent bond to said trunk having a weight ratio with said trunk offrom about 1:9 to about 4:1, said chain being a polymer derived from atleast about 80% of a monomer of a methacrylic ester of the formula CH₂═C(CH₃)COOR, where R is alkyl, aryl, substituted alkyl, substitutedaryl, or substituted alkaryl, and less than about 20% of an acrylic orstyrenic monomer copolymerizable with the methacrylic ester, said chainhaving a Mw of from about 20,000 to 200,000.